Each of the hydraulic drive units in foundry devices of this type usually comprises a stationary cylinder and a piston operating therein, one of the piston rods being firmly connected directly with the pressure plate while the other one is indirectly connected with the counter-pressure plate, viz. through the cross member and the pull and push rods. The two cylinders may be built together so as to constitute a stationary unit and are axially in alignment with each other and with the pressure or squeeze chamber, so that the power transmission to the pressure and counter-pressure plates can take place as simply as possible.
It is moreover known, for instance from German Offenlegungsschrift DE-OS No. 25 28 648 (Henry Wallwork & Company Ltd.) to make use of a drive unit comprising an axially movable cylinder which through pull and push rods is firmly connected to one of the plates, and a piston operating in said cylinder, the piston rod of which supports the second plate. Thus, this provides for saving a cylinder and an associated piston, but on the other hand particular measures are required for the control of the cylinder movement, and it is likewise necessary to make use of movable conduits or hoses for the hydraulic pressure medium.